Process and apparatus for obtaining low purity oxygen by fractionation of air at low temperatures



May 17, 1966 M. SEIDEL 3,251,190

PROCESS AND APPARATUS FOR OBTAINING LOW PURITY OXYGEN BY FRACTIONATION OF AIR AT LOW TEMPERATURES Filed OCT 25, 1962 In venfor MAX SE/DEL Affome ys United States Patent PROCESS AND ArrAnArrJs non OBTAlNING LOW PURITY OXYGEN BY FRACTIONATION or Am AT LOW TEMPERATURES Max Seidel, Munich-Solln, Germany, assignor to Geselh 5 (Zlaims. (or. 62-14 This invention relates to a process and apparatus for obtaining low purity oxygen by low temperature air fractionation in a doublerectification column, and particularly by the use of regenerators as the principal heat exchangers.

The utilization of relatively low purity oxygen (about 45 to 90% oxygen) has greatly increased during recent years. Such oxygen must be produced at a much lower cost than pure oxygen, and should, therefore, be produced by other methods.

In order to reduce the pressure that is necessary for compressing the air that is to be fractionated, it has been well known to flow liquid air with about 70% oxygen enrichment down through a heat exchanger in countercurrent relationship to a rising current of air which is being condensed. At the lower end of the heat exchanger there will be condensation ofa gaseous mixture containing about 21% oxygen, while at the upper end, nearly pure nitrogen will condense. On the opposite side of the heat exchanger, a condensate from the rectifica tion column containing about 70% oxygen will undergo vaporization, the vapors containing about 38% oxygen. At the lower end of the vaporizing side, the remaining liquid will contain about 90% oxygen while the vapors therefrom will contain about 70% oxygen. The condensation as well as the vaporization is, therefore, accompanied by substantial changes in composition of the heat transfer media.

Theoretically this process requires only a very low pressure for the air' to be condensed, but in practice, serious difficulties are encountered because the liquid to be evaporated cannot'be distributed uniformly over the different surfaces of the heat exchanger, and this shortcoming remains present in all other processes in which air fractionation is effected in continuous flow heat exchangers. (German Patents 589,916 and 617,841).

The principal object of the present invention, therefore, is to avoid this diificulty and to devise a process requiring only conventional units of apparatus such as constant level tubular or circulation evaporators, and rectification columns, so that even for widely different loads the necessary heat exchange areas can be accurately calculated on the basis of known records of performance.

Upon further sturdy of the specification and appended claims, other objects and advantages of this invention will become apparent.

This invention starts out with the scheme of imparting to the air that is to be fractionated, the necessary pressure by successive condensations of oxygen-rich and oxygen-poor vapors while in heat exchange relationship with vaporizing oxygen-rich and oxygen-poor liquids, respectively. In the present invention, this is accomplished by subjecting the air that is to be fractionated to partial condensation (the liquid phase containing 32- 39% O and the vapor phase containing 12-45% 0 by volume) by heat exchange with the vaporizing oxygenrich end product which is taken from the low pressure section of the double rectification column. The non- 3,251,190 Patented May 17,- 1966 condensed oxygen-poor part of the air is then conducted into the lower high pressure section of the double rectification column while the condensed oxygen-rich part is passed to the low pressure section of the column at about two to six plates below the place where the liquid from the high pressure section is introduced.

The attached drawing illustrates in schematic flowsheet form the preferred embodiment.

Another feature of this invention is that the condensate, obtained by heat exchange with the vaporizing oxygen-rich end product, is used for washing and cleaning the newly admitted air. This is accomplished by passing the condensate and the air countercurrently through a scrubbing column which is preferably provided with a few rectification trays. The oxygen content of the liquid which collects in the bottom of the scrubbing column is thereby increased up to about 43%, so that this liquid will then be in equilibrium with the oncoming air. The same liquid, preferably after being cleaned by passage through adsorbers, is then introduced at the previously identified place in the low pressure section of the rectification column.

According to a further development of this invention, the evaporator for the liquid end product is operated as a circulation evaporator, from which the circulated but unvaporized portion is drawn off and, preferably after passage over an adsorber, is again returned to the evaporator.

Without further analysis, it is believed that one skilled in the art can readily comprehend and utilize the teachings of this invention. The following description of the preferred embodiment of this invention is, therefore, not to be considered limita'tive of the remainder of the specification and appended claims in any way whatsoever.

Referring in detail to the attached drawing, through the conduit 1 are introduced 102,000 Nmfi/h. of air under an absolute pressure of 3.5 atm. and are passed, first through regenerators 2, 4, 6 and then, during the next phase, through regenerators 3, 5, and 7 to cool the air and free it from H 0 and C0 The resultant 100,000 N-rn. /h. of purified air are then passed through the conduit 8 into the pre-washing column 9, operating at 93 K. and 3.4 atmospheres (absolute), which is provided with rectification trays 10. In this column, the air is washed by the condensate from the subsequently connected separator 22. The discharged wash liqiud which escapes through the outlet pipe 11, and which amounts to about 31,000 -Nm. /h. with an oxygen content of 43% and is in composition equilibrium with entering gaseous air, is conducted into the alternately operated adsorbers 12a, 12b filled with silica gel in which the wash liquid is freed from any carbon dioxide and hydrocarbons that it may contain. The cleansed liquid is then passed through the pipe 13 to the heat exchanger 14 and through expansion valve 17 and inlet opening 18 of the low pressure section '16 of the double rectification column in which it expands down to an absolute pressure of about 1.3 arm.

The washed air which leaves the top of the pre-washing column 9 and which has an oxygen content of about 16%, a temperature of about 92 K., and an absolute pressure of about 3.3 atm., is partially liquefied in the evaporator 20 while in heat exchange with the vaporizing end product having a temperature of about K. and an absolute pressure of about 1.3 atrn., supplied by the pipe 21, and then passes into the separator 22 from which the liquid portion, which contains about 25% oxygen, is drawn off and introduced as a washing liquid into the upper portion of column 9. The gaseous portion of about 69,000 Nm. /h. which still contains about 11% oxygen is delivered through conduit 24 into the lower section 15 of the double rectification column under an abso lute pressure of about 3.2 atm. The liquid which collects in the sump of this column at the rate of about 28,000 Nm. /h., and which contains about 27% oxygen, is conducted by the pipe 25 to the heat exchanger 26 for further cooling, and then passes through expansion valve 27 to the inlet opening 28 of the lowpressure section 16 of the double rectification column.

From the upper part of the high pressure section 15, liquid nitrogen is removed through pipe 29 at the rate of about 19,000 Nm. /h. and is conducted to heat exchanger 30 for further cooling, and is then passed through expansion valve 31 and pipe 32 into the upper part of the low pressure section 16 of the rectifier column in which it serves as a wash liquid. Gaseous nitrogen is taken from the top of high pressure section 15 and is delivered by conduct 33 to heat exchanger 34 in which it is slightly warmed. From there, about 88 volume percent of the slightly warmed nitrogen passes at the rate of about 22,000 Nmfi/h. through conduit 35 to the turbine 36 in which it is expanded while doing work, and is then conducted through heat exchanger 14.

Into heat exchanger 14, additional nitrogen which was taken from the top of the low pressure section 16 and was warmed up by passage through heat exchangers 30 and 26 is then combined at the rate of about 49,000 Nm. /h., thus making a total nitrogen stream of about 71,000 Nm. /h. This nitrogen is conducted through the conduit 37 to regenerators 3 and or 2 and 4 where it is warmed to the temperature of the surroundings and from which it is taken away by conduit 39.

The other portion of the slightly warmed nitrogen from exchanger 34 is passed at the rate of about 3000 Nm. /h. through the pipe 40 into the heat exchanger 41 in countercurrent relation with itself to be warmed to the temperature of the surroundings. It is then compressed in the compressor 42 to about atmospheres, and after being cooled in the heat exchanger 41, is liquified in the heat exchanger 34, is expanded in the control valve 43, and is delivered through the valve 44a either to high pressure section 15 of the fractionating column, or to the stream of liquid nitrogen that is passing from the high pressure section 15 through the heat exchanger 30 to the low pressure section 16.

During the starting operation of the apparatus, the nitrogen which was liquefied in the heat exchanger 34 and expanded in the valve 43 is passed through valve 44b, to the separator 22 and then to the washing column 10, so that the air may be washed before final liquid end product is available from the separator 22.

The 70% liquid oxygen end product which has collected in the sump of the low pressure section 16 of the double rectification column is removed by pipe 21 and delivered to the circulation evaporator 20, where it is vaporized. In the adjacent separator 45, the liquid remaining in the circulating vapors, and which liquid contains about 90% oxygen, is separated from the gaseous portion and is removed from the bottom of the separator by pipe 46 and is passed through the adsorber 47 to the pipe 21 for return to the evaporator. The gaseous end product, consisting of about 29,000 Nm. /h. of 70% oxygen, is removed by pipe 48 from the head of the separator 45, is warmed to room temperature in regenerator 7, and is conducted away by the pipe 49.

The functioning of the regenerators 2/3, 4/5, and 6/7 is periodically interchanged in a known manner. The setting of the valve groups 50a to 50c to the warm sides of the regenerators and the groups of check valves 51a to 510 is determined by the introduced direction of flow to the regenerators.

It is to be noted that evaporator functions to exchange heat between the gaseous phase of the washing column 10 and the oxygen-enriched stream from the bottom of the low pressure column. This heat exchange is an indirect one. The gaseous phase of the washing column 10 which has a lower content of 0 than air is partially condensed, thereby vaporizing the oxygen-enriched liquid from the bottom of the low pressure column 16. The evaporator 20 operates by self-contained circulation, but it is also possible to control the circulation externally.

In comparing this invention with the prior art, the following approximate savings and advantages are obtained, based on a 102000 Nm. charge: To vaporize 70% oxygen by condensing N in a normal type pressure column a pressure of 4.3 atm. abs. would be necessary. It would need 7000 kw. to vaporize that 70% oxygen, based on a 102000 Nm. input of air, whereas only 6000 kw. are needed if the pressure is only 3.5 atm. abs.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Consequently, such changes and modifications are properly, equitably, and intended to be, within the full range of equivalence of the following claims.

What is claimed is:

1. A process for producing low purity oxygen by the low temperature fractionation of air in a double rectification column having a high pressure zone and a low pressure zone, which process comprises:

(a) conducting compressed air to be fractionated through regenerators and cooling it to a low temperature;

(b) conducting the cooled gaseous air to a washing zone and washing the air with liquid air enriched in oxygen;

(-0) conducting the liquid fraction from the washing zone to an intermediary section of the low pressure zone of the double rectification column;

(d) conducting all of the gaseous fraction from the washing zone to a heat exchange zone, in indirect heat exchange relationship with a liquid comprising bottoms from said low pressure zone to partially condense said gaseous fraction-into a liquid fraction enriched in oxygen and a vapor fraction depleted in oxygen;

(e) conducting the liquid fraction of said partially condensed gaseous fraction to the washing zone as sole washing liquid;

(f) conducting the vapor fraction of said partially condensed fraction to the high pressure zone of the double rectification column.

2. The process for producing low purity oxygen, according to claim 1, further comprising the steps of mixing oxygen-rich liquid from the .bottom of the low pressure zone of the double rectification column with a liquid still richer in oxygen, conducting the resultant mix-.

ture to said heat exchange zone, thereby partially vaporizing it, and con-ducting the remainder of the liquid fraction through an adsorbent and then re-circulating the said remainder through the heat exchange zone.

3. The process for producing low purity oxygen, according to claim 1, further comprising the steps of withdrawing gaseous nitrogen from the top section of the high pressure zone of the double retification column, Warming the nitrogen in countercurrent heat exchange with high pressure nitrogen, dividing said warmed nitrogen into two fractions, compressing the first fraction to a higher pressure, l-iquefying it in said countercurrent heat exchange with gaseous nitrogen from said high pressure zone, throttling and charging it into the top section of the low pressure zone of said double rectification column, while conducting the second fraction to an expansion zone thereby expanding the said fraction to produce external energy.

4. A process for producing low purity oxygen by the low temperature fractionation of air in a double rectifi- 5 cation column having a high pressure zone and a low pressure zone, which process comprises:

(a) conducting compressed air to be fractionated through regenerators and cooling it to a low temperature;

(b) conducting the cooled gaseous air to a washing zone and washing the air with liquid air enriched in oxygen;

(c) conducting the liquid fraction from the washing zone to an intermediate low pressure zone of the double rectification column;

(cl) conducting the gaseous fraction from the washing zone to a heat exchange zone thereby partially condensing said gaseous fraction; V

(e) conducting the liquid fraction of said partially condensed gaseous fraction to the washing zone as a rinser;

(f) conducting the gaseous fraction of said partially condensed fraction to the high pressure zone of the double rectification column;

(g) withdrawing gaseous nitrogen from the top section of the high pressure zone of the double rectification column;

(h) warming the nitrogen in countercurrent heat exchange with high pressure nitrogen;

(i) dividing said warmed nitrogen into two fractions;

(j) compressing the first fraction to a higher pressure;

(k) liquefying it in said countercurrent heat exchange with gaseous nitrogen from said high pressure zone;

(1) throttling and charging it into the top section of the low pressure zone of said double rectification column; and

(m) while conducting the second fraction to an expansion zone thereby expanding the said fraction to produce external energy.

5. Apparatus for producing low purity oxygen, which apparatus comprises: regenerators for the cooling and cleansing of air, each of the regenerators having a cold Cir the liquid inlet of said prewash means via said first separating means, and to said cold terminals of the regenerators via said second separating means, and the liquid outlet of said prewash means being connected to the top section of the said intermediary low pressure zone congruous to the overhead column, compressing means and expanding means ifOI gaseous nitrogen, each of said means covering a high pressure and a low pressure zone, the high pressure zone of said compressing means being connected to the top section of the overhead column, the low pressure zone of said compressing means being connected to the top section of the lower unit of said double rectification column and to the high pressure zone of said expanding means, and the low pressure zone of said expanding means being connected to the cold terminals of the regenerators.

References Cited by the Examiner UNITED STATES PATENTS 10/1946 Van Nuys 6230 X 2,615,312 10/1952 Yendell 62-15 X 2,619,810 12/1952 Rice 62-14 2,664,719 1/1954 Rice 62-14 X 2,785,548 3/1957 Becker 6226 2,825,212 3/1958 Linde 6213 X 2,873,583 2/1959 Potts 6214 3,065,607 11/1962 Schilling 6229 X NORMAN YUDKOF-F, Primary Examiner 

1. A PROCESS FOR PRODUCING LOW PURITY OXYGEN BY THE LOW TEMPERATURE FRACTIONATION OF AIR IN A DOUBLE RECTIFICATION COLUMN HAVING A HIGH PRESSURE ZONE AND A LOW PRESSURE ZONE, WHICH PROCESS COMPRISES: (A) CONDUCTING COMPRESSED AIR TO BE FRACTIONATED THROUGH REGENERATORS AND COOLING IT TO A LOW TEMPERATURE; (B) CONDUCTING THE COOLED GASEOUS AIR TO A WASHING ZONE AND WASHING THE AIR WITH LIQUID AIR ENRICHED IN OXYGEN; (C) CONDUCTING THE LIQUID FRACTION FROM THE WASHING ZONE TO AN INTERMEDIARY SECTION OF THE LOW PRESSURE ZONE OF THE DOUBLE RECTIFICATION COLUMN; (D) CONDUCTING ALL OF THE GASEOUS FRACTION FROM THE WASHING ZONE TO A HEAT EXCHANGE ZONE, IN INDIRECT HEAT EXCHANGE RELATIONSHIP WITH A LIQUID COMPRISING BOTTOMS FROM SAID LOW PRESSURE ZONE TO PARTIALLY CONDENSE SAID GASEOUS FRACTION INTO A LIQUID FRACTION ENRICHED IN OXYGEN AND A VAPOR FRACTION DEPLETED IN OXYGEN; (E) CONDUCTING THE LIQUID FRACTION OF SAID PARTIALLY CONDENSED GASEOUS FRACTION TO THE WASHING ZONE AS SOLE WASHING LIQUID; (F) CONDUCTING THE VAPOR FRACTION OF SAID PARTIALLY CONDENSED FRACTION TO THE HIGH PRESSURE ZONE OF THE DOUBLE RECTIFICATION COLUMN. 